Compound boost not as expected

[craigory]

Heres a pull from idle in 3rd gear.


https://www.flickr.com/gp/157274311@N07/41zU9e

Top left gauge- 18psi compounded boost, there by around 3000rpm. My gauge under reads cause its crap. (Shows 15psi)

Bottom left gauge- Turbo only, blowing 8-9 psi into supercharger

Top right gauge- Exhaust back pressure, reading 11 maybe 12 psi.

Seems to work half decent for me..??
I could only imagine how long that turbo would take to spool with that size exhaust turbine with no supercharger in there.. Im guessing the turbo thinks its bolted to a 3.3 to 3.5 liter engine.

Waiting on another boost control solenoid so i can get this up to around 25-28psi Compounded boost.

[craigory]

I'm trying to compound boost beyond exhaust turbine back pressure. Have more pressure at the inlet valve than back pressure at the exhaust valve.. most turbo engines will create way more back pressure than boost..

Turbo only creates more backpressure than boost with really small turbines. Holset 1C with huge turbine is make for all-time full throttle use with diesels, so it's exhaust backpressure is nowhere near boost pressure. You can't operate at full throttle long periods with small exhaust turbine with more back pressure than boost, it will melt engine soon.

And roots blower is only a fixed-volume air pump, it isn't effective as second phase charging. Usually when using roots blower combined with turbo it is made so that roots blows engine at low rpm range when big-turbine turbo isn't spooled up yet, making boosted power before turbo and making that turbo come to live at much lower rpm range than without blower assistance.

Those Holset diesel turbos are usually tuned to operate at high boost pressures so they might not be in best operating range with 2.3litre engine with 8.5 psi boost, if that compressor has split intake it probably is still reverse-flowing until near 1bar boost(14psi or so) to keep turbine from surging - small roots blower between it and engine probably just makes that situation worse.

Its my understanding from everything ive read that nearly all turbocharged engines will create more back pressure than boost. Except for highly tuned race engines that operate in a high rpm bracket, with perfectly sized compressor and turbine housings and little restriction up or downstream...
Turbo cams have little overlap, not to stop boost escaping but to keep the exhaust gasses from reversing back into the cylinder.

The roots blower is indeed a fixed volume pump.. just takes what ever is presented to its inlet and compresses it further. That volume could be atmo air or already compressed air, it doesn't know or care.
(Credit to warpspeed for this, its my main argument when someone mentions a roots after turbo is a restriction)

As mentioned above. Turbo is tricked into thinking its on a bigger cc engine so its in a different part of its efficiency map.

I'm no engineer just a man in his garage having fun.
Im getting results but always eager to learn.

[TimIacobucci]

Turbo only creates more backpressure than boost with really small turbines. Holset 1C with huge turbine is make for all-time full throttle use with diesels, so it's exhaust backpressure is nowhere near boost pressure. You can't operate at full throttle long periods with small exhaust turbine with more back pressure than boost, it will melt engine soon.

I've put some pretty big turbos on some pretty small high revving engines and measured backpressure. More recent being 600+ hp 2.0l Honda spooling a 67mm turbo with a large turbine housing after 6000 rpm, still a good power band past 10,000, still more backpressure than boost. Even breaking even is doing amazing turbo only in my book. I think you need a very serious setup to actually fully operate continuously to redline with less backpressure than boost with a bypassed (gated) turbine. I would very much like to like to test out variable turbines.

Those Holset diesel turbos are usually tuned to operate at high boost pressures so they might not be in best operating range with 2.3litre engine with 8.5 psi boost, if that compressor has split intake it probably is still reverse-flowing until near 1bar boost(14psi or so) to keep turbine from surging - small roots blower between it and engine probably just makes that situation worse.

Before you get to the blower part I agree with you. I have actually run this turbo on this engine without a blower, it's a dog until 25+ psi, fortunately the crossover to that point happens fast when it does come on. Fast enough I actual spun the short wheelbase fox body it was in before I got a handle on the power curve. It was just full throttle na low compression 90hp 2.3l lag lag lag BOOM tire annihilation. Not much relation between throttle movement and power output.

Watch his video though, that blower is very quickly making 10 psi. That's near a 1.7 pressure ratio. The turbo thinks it's on a 4.0L engine. That puts it way out into a better flow range at the same boost very quickly compared to a 2.3. More importantly it has actual throttle response and control and midrange power, that was 3rd from IDLE. I agree this turbo seems to have much better efficiency at higher boost but I believe that is the ultimate intention with this setup once the bypass issues and controls are sorted first and I think he has a handle on that now. Also look at the backpressure gauge, this setup sans turbo is not below boost pressure.

And roots blower is only a fixed-volume air pump, it isn't effective as second phase charging. Usually when using roots blower combined with turbo it is made so that roots blows engine at low rpm range when big-turbine turbo isn't spooled up yet, making boosted power before turbo and making that turbo come to live at much lower rpm range than without blower assistance.

Generally I don't care to speculate too much as would I rather wait until it goes on a track or dyno and assess things from measurements and datalogs. I would very much like to test the differences between variations of the blower to turbo boost ratio and its relation to backpressure and power output.

I will speculate a bit here from reading I've done on successful twincharge setups though. I think there are arguments both ways for bypassing vs compounding the blower and like most things i believe the devil will be in the details. I believe there are a few reasons most twincharge systems using a factory style supercharger usually make more power turbo only. The blower is generally too restrictive and not sized large enough or driven to operate efficiently at high rpm or most especially not given adequate charge cooling after the blower if it has any at all. It was engineered for response and to give the full power assist the system needed. If you size the blower up and don't drive it as hard it can flow pretty well at higher rpms. Blower design is even relevant here too, newer factory tvs setups illustrate this very well, even at factory settings they can be very efficient at high rpm now. Most old factory Eatons like the terminator cobras were off the damn compressor map at redline with the stock drive ratio. Combine this with limited space to cool the inefficient compression then yes, I agree that compounding that is not ideal.

With the throttle in the stock location and warpspeed style wastegate blower bypass it opens the door for much better charge cooling as craigory has implemented. Combine this with a blower sized and driven to work well at higher rpm and only hard enough to light off the turbo and the compound result is very different.

Another problem is making the turbo too small. A turbo sized well for a 2.3 on 4.0l engine is not going to be nearly as efficient as it would on the 2.3l. If you compound it at an already inefficient flow rate into an inefficient factory blower you've got a good recipe for a whole lot of heat and not much else.

I don't think the practical limit of a too big turbo on a twincharge setup has been found yet to my knowledge. I would love to hear about it if someone has done it.

Then there's cam selection. With a favorible backpressure you can run very different cams like a more race oriented turbo only lag monster, but you can also benefit from it during the entire mid range as the blower is a nice anti reversion device. I'm not sure how much blower boost is actually needed for this benefit though. It is very likely turbo /turbine size and cam dependent. Eliminating reversion also opens the tuning window in regards to detonation dramatically. I have experienced this also with small vs big turbos on the same engine, lower egts and backpressure always raises knock threshold, seemingly contradictorily as you make more torque and power with the bigger turbo which is also when you can run more overlap and these benefits all snowball as the turbo and turbine get bigger. You just need to rev the hell out of the engine and turn the boost up quite a bit for it to work, in this system the blower is doing that job for you.

[Wolf_Tm250]

Sorry to the OP, as I will not help his issue, but go Off Topic.
Just wanted to say that it's very difficult for me to understand why you should need a compound system nowadays in a well geared car and with the new Garrett GTX2 twin scrolled.
Off Topic End.

[craigory]

Sorry to the OP, as I will not help his issue, but go Off Topic.
Just wanted to say that it's very difficult for me to understand why you should need a compound system nowadays in a well geared car and with the new Garrett GTX2 twin scrolled.
Off Topic End.

You are probably right. But both my power adders cost me 500 canadian in total. How much is that garrett?

But budget aside its always been about changing the characteristics of flow of a turbo only motor, that's it really.

[englertracing]

Sorry to the OP, as I will not help his issue, but go Off Topic.
Just wanted to say that it's very difficult for me to understand why you should need a compound system nowadays in a well geared car and with the new Garrett GTX2 twin scrolled.
Off Topic End.

Maybe some people enjoy the immediate response of positive displacement, especially drivers who partake in throttle steer

[TimIacobucci]

Sorry to the OP, as I will not help his issue, but go Off Topic.
Just wanted to say that it's very difficult for me to understand why you should need a compound system nowadays in a well geared car and with the new Garrett GTX2 twin scrolled.
Off Topic End.

Result was just over 700ft/lbs torque at 5500rpm, no lag at all and a very healthy spread of torque from 2500rpm up to 7800rpm. Power at the flywheel estimated at 700hp. Estimated because the dyno only gave wheel figures and wasn't setup for coast-down losses. Going by the previous visits to a different dyno, we have estimated 100+ hp and torque losses through the 4wd system.

It's incredible to drive now. From 2000rpm it pulls hard even in 5th and picks up a fair pace very quickly! Overall i think a very worthwhile modification as the car is very very quick on the hillclimb tracks. No waiting for the turbo to spool on the exit of the hairpins, it's instant torque at any revs and speed!

The last few events i've suffered clitch slip so a triple plate Superclutch has been fitted. There's also been the odd transmission breakage due to fatigue. I stripped first gear on the line and then twisted the centre diff output shaft at Doune. Both breakeages have evidence of class

From this thread, https://www.eng-tips.com/viewthread.cfm?qid=267889

That's on a 2.0l evo for reference.

Let me know which 700hp gtx does that.

[ptuomov]

Sorry to the OP, as I will not help his issue, but go Off Topic.
Just wanted to say that it's very difficult for me to understand why you should need a compound system nowadays in a well geared car and with the new Garrett GTX2 twin scrolled. Off Topic End.

For the same reason why some people would prefer larger displacement engines with turbo. This twin charging is just making the engine bigger plus making the dealing with the exhaust blowdown interference a little easier, as the intake pressure that the cams & valves see goes up and the exhaust path to the turbine doesn't need to be super short.

[craigory]

One other thing I really like about this is if im part throttle at higher rpm the turbo is already spooling into the charger and its then bypassed. Shut that bypass and its full boost right there.
Turbo only is dead heading against a closed BOV and part closed throttle body. Wot and it has to spool up..

[craigory]

I also believe that the charger keeps the turbo from freewheeling and over spinning it.

[gruntguru]

Then there's cam selection. With a favorible backpressure you can run very different cams like a more race oriented turbo only lag monster, but you can also benefit from it during the entire mid range as the blower is a nice anti reversion device.

The blower is only an anti-reversion device to the extent that it enables a more favorable pressure differential across the engine. If exhaust back pressure is higher than intake manifold pressure, reversion during overlap is not reduced by the presence of a positive displacement supercharger.

[TimIacobucci]

That's why you pick an enormous turbine housing, look at the posted video to see an impressive boost/backpressure ratio. If the turbo is sized such that without the blower it's already close to 1:1 it really shouldn't take much from the blower to achieve a favorable differential.

[craigory]

e.

The blower is only an anti-reversion device to the extent that it enables a more favorable pressure differential across the engine. If exhaust back pressure is higher than intake manifold pressure, reversion during overlap is not reduced by the presence of a positive displacement supercharger.

Your first statement is basically the whole point of this set up.

Your next point is what you should try and fix if that happens.. Change turbine housing, change charger gearing ratio. But if your not getting the desired pressure differential across the engine, your just pointlessly beating up the boost with the roots. Adding massive amounts of heat and wasting your time. .
Compound twincharging adds a lot of heat. But if you can blow all the residual exhaust gasses out and fill with elevated temp boost I'm sure your still winning.. Elevated boost temps or lingering exhaust gases which is worse? Defo the latter imo.

[englertracing]

Sorry to the OP, as I will not help his issue, but go Off Topic.
Just wanted to say that it's very difficult for me to understand why you should need a compound system nowadays in a well geared car and with the new Garrett GTX2 twin scrolled.
Off Topic End.

https://youtu.be/5HlqEkJ3VGM?t=3m37s
it makes a 2 liter 4 cylinder run like this

and smoke 6 cylinder turbo motors...
https://youtu.be/u9FpUeoiJXg?t=7m24s
watch the porsche and then the lancia follows, Ill take the lancia.....

[craigory]

Was the lancia not a parallel type system? Not actually compound?